Dear Statalist,
Before applying a repeated ANCOVA on my own data, I tried to run the example
proposed by Tabachnick and Fidell (Computer-assisted research design and
analysis, 2001) on page 417 (Table 8.13). The data set is a mixed ANCOVA
with a single covariate (Easy) measured once for each case. The
randomized-groups IV i type of music (labeled Music, 2 levels) and the
repeated-measures IV is time of day (4 levels, labeled T1 through T4).
The data are below :
Music Easy T1 T2 T3 T4
1 6 8 7 5 3
1 6 9 7 5 1
1 4 8 5 4 1
1 3 6 6 4 2
2 6 2 4 8 10
2 7 6 5 9 10
2 3 2 4 6 10
2 5 3 5 6 9
The data set has been arranged for use in Stata:
music easy y time id
1 6 8 1 1
1 6 9 1 2
1 4 8 1 3
1 3 6 1 4
2 6 2 1 5
2 7 6 1 6
2 3 2 1 7
2 5 3 1 8
1 6 7 2 1
1 6 7 2 2
1 4 5 2 3
1 3 6 2 4
2 6 4 2 5
2 7 5 2 6
2 3 4 2 7
2 5 5 2 8
1 6 5 3 1
1 6 5 3 2
1 4 4 3 3
1 3 4 3 4
2 6 8 3 5
2 7 9 3 6
2 3 6 3 7
2 5 6 3 8
1 6 3 4 1
1 6 1 4 2
1 4 1 4 3
1 3 2 4 4
2 6 10 4 5
2 7 10 4 6
2 3 10 4 7
2 5 9 4 8
I entered the following command :
anova y easy music/id|music time time*music time*easy, rep(time) cont(easy)
I got the same same results than reported by Tabachnick and Fidell (using
SYSTAT GLM) except for the covariate :
SS df MS F P
Stata .125 1 .125 0.18 0.6921
Tabachnick 11.327 1 11.327 15.96 0.01
and Fidell
How can this difference be explained ?
Secondly, I tried to test the homogeneity of regression slopes, i.e. the
interaction between the covariate Easy and the factor Music by
anova y easy easy*music music/id|music time time*music time*easy, rep(time)
cont(easy)
I got :
SS df MS F P
easy | .125 1 .125 0.18 0.6921
easy*music | 0.00 0
music | .491276672 1 .491276672 0.69 0.4433
id|music | 3.5483871 5 .709677419
Why the df of easy*music is 0 ?
Sincerely yours,
Xavier VIVIAND
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